An HMD (Head Mount Display) is a type of virtual image optical systems. An optical system is an image display device to allow a user to see an enlarged virtual image by forming a focus so that an image light generated from a position very near to an eye can be configured on a virtual large-sized screen at a remote distance using an accurate optical device. Although an optical system for HMD was originally developed for military purpose, the optical system for HMD is currently used in various fields including aerospace, video games, practical training and leisure fields as the high performance and miniaturization of computer systems are drastically advanced and rapid development of display devices provide a wearable computer concept wearable on a body.
Inter alia, a head mounted display device is used with a method in which an image of a display device can be viewed by a user and the head mounted display device is also mounted with a camera to photograph a sight viewed by a user so that the photographed sight is provided to a user. At this time, however, the camera cannot be mounted at a place in front of an eye of a user such that the camera is inevitably mounted at a lateral surface of an eye. Thus, when the camera is mounted to face a front side, there may be generated with a problem where a vision field viewed by a user of the head mounted display device and a photographed scope photographed by a camera cannot be matched.